1. Field of the Invention
This invention concerns a method of and an apparatus for measuring elongation of a test specimen in a contactless manner without attaching reference lines (or reference points) to a test specimen and, more in particular, it relates to a method of and an apparatus for measuring elongation by utilizing the amount of movement of fringes in speckle patterns that appear in the reflection light of laser beams irradiated to the test specimen.
2. Related Art Statement
As the means for measuring the elongation of a test specimen in a contactless manner, there has been a method of attaching reference lines at two points in the direction of elongation of the test specimen, photographing respective reference lines by two television cameras individually which are recognized by a computer as marks for each of them on a television screens, calculating the moving amount of the marks on the television screens that move along with the elongation of the test specimen, controlling the positions of the two television cameras such that respective marks always situate at predetermined positions displayed on the television screens and measuring the elongation rate based on the difference of the distance in the two television cameras before applying a tensile load and upon rupture of the test specimen.
Further, as the means of measuring the elongation of a test specimen without attaching reference lines in a contactless manner, there have been proposed various methods of and apparatus for measuring elongation in the contactless manner by irradiating a laser beam having an extension for a predetermined length in the direction of exerting a tensile load to the surface of a test specimen and recognizing the reflection light from the region corresponding to reference spots at two positions of the test specimen by electric signals as varying levels of brightness in the speckle pattern, and calculating the elongation of the test specimen based on a correlation function of the electric signals for the speckle pattern by using the output from the image sensor, for example, as disclosed in Japanese Published Unexamined Patent Application Sho 59-52963, Japanese Published Examined Patent Application Sho 61-27681 and Japanese Published Unexamined Patent Application Hei 7-4928.
However, the former method, that is, a method of using the television cameras involves a fetal drawback that reference lines have to be attached to the test specimen. That is, the method of attaching the reference lines includes a method of scratching a test specimen, a method of appending a seal on a test specimen, a method of applying a mark with ink or the like, a method of clipping the test specimen with binders at reference positions. However, the measuring method of appending the reference lines involves problems such as of giving undesired effects on the strength of the test specimen, peeling of the seal, elongation of the reference lines per se making the reference base obscure and, further, causing chemical change to the test specimen by the ink of the reference, which hinders accuracy and efficiency of the test.
On the other hand, the latter method, that is, each of the existent methods of measuring the elongation by utilizing the speckle pattern is adapted for optoelectronically converting reflection light obtained by the irradiation of the laser beam to a portion of a test specimen to be measured by way of an image sensor or the like to obtain electric signals in accordance with the speckle pattern and determining the amount of movement of the speckle pattern based on the correlation function of signals before and after elongation of the test specimen. That is, since the speckle pattern recognized by the image sensor is converted and used as electric signals, it is difficult to attain high measuring accuracy unless precise optical parts and conversion devices are used.
Further, in the existent measurement for elongation by utilizing the speckle pattern is adapted to previously store the speckle pattern in the direction of the elongation of the test specimen before deformation (entire spot pattern) as electric signals and determine the amount of movement of the speckle pattern by continuously deciding the correlation function for the speckle pattern upon deformation relative to the electric signals. Therefore, if the test specimen and the laser beam are laterally displaced relative to each other during measurement, the speckle pattern per se as a reference base becomes inaccurate to bring about a problem that there is no reliability at all for the measured value. Particularly, since the tensile load is applied on the test specimen during measurement and slight lateral deviation is inevitable, this problem is fetal for the measurement.
Further, as a general problem in the existent measurement for elongation, since elongation at break is determined based on the distance between the reference lines before applying the tensile load and the final distance between the reference lines upon rupture of the test specimen by the tensile load, tensile load is applied till rupture occurs in the test specimen. However, since the test specimen scatters upon rupture depending on the material to bring about a danger, an operator has to take a safe shelter after setting the test specimen to an elongation measuring apparatus, which gives a bar to the operation efficiency.
Accordingly, it is a first object of this invention to provide a method of measuring elongation in a contactless manner not requiring setting of reference points to a test specimen, capable of obtaining an accurate measured value without using precise optical devices or conversion devices and irrespective of lateral displacement of the test specimen and further capable of being automated, as well as an apparatus for measuring elongation for practicing the method.
A second object of this invention is to provide a method of measuring the elongation described above, capable of utilizing instruments for image processing, calculative operation and control used so far in existent apparatus using television cameras as they are, as well as an apparatus for measuring the elongation for practicing the method.
A third object of this invention is to provide a method of measuring elongation capable of detecting the state just before rupture of a test specimen and determining a measured value for elongation substantially equal with that upon rupture without fracturing the test specimen, as well as an apparatus for practicing the method.
The first object of this invention can be attained in accordance with a first method of measuring elongation in a contactless manner while applying a tensile load to a test specimen, the method comprising irradiating laser beams from two sensor sections each integrally having a laser projector and a CCD camera to prescribed reference line positions at two positions set along the direction of elongation on the test specimen respectively, photographing the laser reflection lights by the CCD cameras respectively as speckle patterns each comprising a plurality of fringes, recognizing fringes at predetermined positions of the speckle patterns as targets corresponding to the respective prescribed reference line positions at the two positions on the coordinate of the CCD camera screens, detecting the moving amount on the pixel unit basis of the coordinate of the target fringes in the respective speckle patterns that move in accordance with the elongation of the test specimen, conducting tracking control for the two sensor sections along the direction of elongation of the test specimen by the detection signals on every movement such that the target fringes are always situated at the predetermined positions of the screens and determining elongation information for the test specimen in view of the distance between the two sensor sections, the amount of respective movement detection signals or the moving amount on the pixel unit basis of the respective target fringes.
The first method of this invention can be practiced in accordance with a first apparatus for measuring elongation of a test specimen, comprising a pair of sensor sections each having a laser projector, a CCD camera and irradiating laser beams to prescribed reference line positions at two positions along the direction of elongation of a test specimen set to a tensile strength tester, respectively, and photographing the laser reflection lights by the CCD cameras, respectively, as speckle patterns each comprising a plurality of fringes, a pair of sensor driving sections for movably supporting the pair of the sensor sections along respective guides and moving the sensor sections along the guides in accordance with input signals, respectively, a control section recognizing the fringes at predetermined positions of the speckle patterns photographed by the CCD cameras of the sensor sections as target fringes corresponding to the prescribed reference line positions, respectively, detecting the moving amount of the target fringes that move in accordance with the elongation of the test specimen on the pixel unit basis of the photographed images and controlling the pair of sensor driving sections by the detection signals such that the target fringes are situated at the predetermined positions of the speckle patterns, respectively, and an operation section for determining the elongation information for the test specimen in view of the distance between the two sensor sections, the amount of the respective movement detection signals or the moving amount on the pixel unit basis of the target fringes.
The second object of this invention can be attained also in accordance with a second method of measuring elongation in a contactless manner while applying a tensile load to a test specimen, the method comprising irradiating laser beams from two sensor sections each integrally having a laser projector and a CCD camera to prescribed reference line positions at two positions set along the direction of elongation on the test specimen, respectively, photographing the laser reflection lights by the CCD cameras respectively as speckle patterns each comprising a plurality of fringes, recognizing fringes at predetermined positions of the speckle patterns as targets corresponding to the respective prescribed reference line positions at the two positions, image converting target fringes into marks at predetermined positions of two television camera screens respectively, detecting the moving amount on the pixel unit basis of the marks of the television screens that move in accordance with the elongation of the test specimen, controlling the movement of the two sensor sections respectively along the direction of elongation of the test specimen by the detection signals on every movement such that the marks are always situated at the predetermined positions of the televisions screens, and determining the elongation information for the test specimen in view of the distance between the two sensor sections, the amount of the respective mark movement detection signals or the moving amount on the pixel unit basis of the television image marks, respectively.
The second method of this invention can be practiced in accordance with a second apparatus for measuring elongation of a test specimen, comprising a pair of sensor sections each having a laser projector and a CCD camera and irradiating laser beams to prescribed reference line positions at two positions along the direction of elongation of a test specimen set to a tensile strength tester, respectively, and photographing the laser reflection lights by the CCD cameras respectively as speckle patterns each comprising a plurality of fringes, a pair of sensor driving sections for movably supporting the pair of sensor sections along respective guides and moving the sensor sections along the guides in accordance with input signals, respectively, a pair of image conversion devices disposed corresponding to the sensor sections, respectively, and photographing the target fringes photographed by the CCD cameras of the sensor sections as marks at predetermined positions on the television camera screens, respectively, a control section recognizing the marks photographed by the television cameras 25 and 26 as reference lines corresponding to the prescribed reference line positions respectively, detecting the moving amount of the marks that move in accordance with the elongation of the test specimen on the pixel unit basis of the television screen and controlling the pair of the sensor driving sections by the detection signals such that the marks are situated at the predetermined positions of the television screens and an operation section for determining the elongation information for the test specimen in view of the distance between the two sensor sections, the amount of the respective mark movement detection signals or the moving amount on the pixel unit basis of the television image marks, respectively.
The third object of this invention can be attained by a third method embodied from each of the first or second measuring method described above, wherein stopping of images or distortion of images of speckle patterns appearing just before rupture of the application of the test specimen is sensed to stop the tensile load on the test specimen.
The third method of this invention can be practiced by a third apparatus embodied from each of the first or second apparatus described above, which further comprises a load control device that senses stopping of the images and disturbance of the images of the speckle patterns of the CCD cameras appearing just the before the rupture of the test specimen and outputting a tensile load stopping signal for the test specimen.